Microphone sound filter



Jan. 2, 1951 Q s. A. CALDWELL, JR 2,536,261

MICROPHONE SOUND FILTER Filed Dec. 20, 1947 Zhwcntor Ja m- ,4. (filin /Z17.

Qttorneg latented Jan. 2, 1951 MICROPHONE SOUND FILTER Stephen A. Caldwell, Jr., Haddon Heights, N. J., assignor to Radio Corporation of America, a

corporation of Delaware Application December 20, 1947, Serial No. 792,969

9 Claims. 1

The present invention relates to speech microphones of the close talking type, and has for its primary object to provide an improved speech microphone which effectively reduces acoustical feedback and noise pickup while maintaining a high degree of speech sensitivity.

It is a further object of this invention, to provide an improved speech microphone having a high acoustical sound input impedance and means for maintaining said high impedance during close talking operation thereof.

It is also a further object of this invention, to provide an improved speech microphone provided with a high impedance sound input filter system and shaped to prevent standing waves externally thereof, whereby acoustical feedback and noise pickup by the microphone may greatly be reduced without impairing the normal speech input characteristic thereof.

In accordance with the invention, the high acoustical impedance of relatively small openings providing an effective speech input area of reduced size for a microphone, is utilized to increase the ratio of speech level input to feedback and noise level input, the effective area of the microphone diaphragm being determined thereby and placed at a point where the sound level for speech is relatively high.

The invention will, however, be better understood from the following description, when considered with reference to the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawings:

Figure l is a top view, on a reduced scale, of a speech microphone embodying the invention;

Figure 2 is a view in elevation, and partly in section, of the microphone of Figure 1 showing further details of the construction thereof in accordance with the invention; and

Figure 3 is a line diagram illustrating a principle of operation of the microphone of Figures 1 and 2.

Referring to Figures 1 and 2, 5 is a cylindrical cup-shaped microphone housing or casing in which is mounted any suitable speech microphone unit 6 indicated in dotted outline.

The microphone casing 5 is closed by a dome or bullet shaped hollow cap or casing l which seats at its base end over the open end or face of the casing 5, as indicated, and terminates at its forward end or tip in an opening in which is seated a flanged mouthpiece 8.

The mouthpiece 8 is cylindrical or annular in shape and comprises an external, annular, forwardly projecting flange 9 surrounding the tip end of the cap I, and an inner central wall l0 integral therewith, seated in the end of the cap and provided with a plurality of spaced openings ll of any suitable shape providing high impedance sound communication between the exterior of the cap 1 and the hollow interior thereof.

The flange 9 is of sufficient depth to provide 1 a shallow central air space or cavity l2 within the mouthpiece, between the inner wall It and an outer screen disk i3, which is pressed into the open end of the mouthpiece and grips the flange 9. The screen is of relatively fine mesh, such as 300 mesh for example, and the flange 3 is provided with a plurality of small vent holes or channels 14 extending radially therethrough at spaced points between the air chamber l2 and the sides of the mouthpiece.

The hollow interior of the dome or bullet shaped cap or casing i is packed in an outer area adjacent its outer walls, with a suitable acoustic damping material indicated at 15, and centrally in an inner area, less densely, with similar damping material as indicated at IS, the inner and the outer areas being provided with a line of demarcation in the form of a cylindrical screen I? extending longitudinally of the cap I from a forward point in rear of the mouthpiece, through the cap to a central orifice or opening I8 provided in a cylindrical plug [9 which fits into the open end of the microphone casing 5 within the cap I at the base or rear thereof as indicated.

The plug is flared rearwardly to provide a relatively wide internal chamber or space 20 providing acoustical capacitance in front of the microphone [5 which is coupled to the larger, damped, chambered interior of the cap "I through the acoustical impedance or resistance of the opening I8.

The mouthpiece 8, the microphone casing 5 and the cap I, together with the internal plug I9 may be made of any suitable material. However, at present these elements are preferably constructed of suitable molded plastic material as indicated.

Referring now to Figure 3 along with Figures 1 and 2, if it is assumed that S is a point source of sound which has a directional characteristic whereby a major portion of the sound energy radiated lies in the angle ASB, a diaphragm placed at AB of diameter A-B will have impinged on it all the energy radiated by S in the angle ASB and an amount of sound energy, in

pinging on both diaphragms is nearly the same,.

the feedback or noise pickup by the small diaphragm is reduced by the ratio of the area of the small diaphragm to the area of the large diaphragm.

A microphone embodying the invention, as shown for example in Figures 1 and 2, utilizes the above described principle of operation, in that an effective diaphragm for the microphone 6 of relatively small area is provided at the mouthpiece 8. Because of the high acoustic impedance presented to sound waves by the small openings i i, and the fact that the only sound energy which actuatesthe microphone must come through the openings, the effective diaphragm area is at the mouthpiece and the openings, with the microphone 6 facing the chamber 20 and the openings ll through the interior of the cap 1.

This effect has been substantiated by experiment. It has been found that the variation of signal output of the microphone with variation of distance of the openings from a small sound source of constant output very nearly followed the inverse square law. The openings l I may be one or more in number and may be of any suitable shape, provided that the total effective area at the mouthpiece is relatively small with respect to the diaphragm area of the microphone 6.

In operation, the mouthpiece 8 is held in a position for close talking, whereby a concentration of high level sound is provided directly at the small area entrance. The high level speech therefore enters the microphone cap I and actuates the microphone by the fact that it overcomes by pressure the high impedance of the mouthpiece 8, while at the same time external feedback and noises do not gain entrance by reason of the fact that the sound pressure at the mouthpiece for such feedback and noise is relatively low.

The mouthpiece unit having an inner wall perforated with small openings for high impedance sound inlet, an outer relatively fine mesh wind screen spaced therefrom, and means for laterally or radially venting the space or cavity between the inner wall and outer screen provides a noise filter for high pressure sound or speech input which is of low cost and simple compact construction.

It will further be seen that the fine mesh wind screen l3 provides a protective covering for the openings ll while preventing the entrance of foreign material or particles which might block the openings, without impeding the sound input at high pressure. The lateral vent holes l4 further permit the smooth flow of air from the chamber or cavity l2 externally of the mouthpiece to reduce turbulence or wind eifects and noises while the microphone is in use for close talking operation.

Therefore, primarily, the wind screen and filter atthe mouthpiece serve to reduce noises. Without the wind screen, small vortices are formed across the outer face of the wall H] and the openings H and create noise. With the wind screen,

the vortices form outside the screen as pressure builds up in the space or cavity l2. The vents l4 permit free air flow, to reduce the noise by reducing turbulence at the high impedance input openings.

For best results, for close talking operation, it has been found that the size of the opening in the mouthpiece, that is the size of the perforated area in the wall it should conform approximately to the size of the sound source, such .as a speakers mouth.

The internal acoustic damping material within the cap 7 prevents the creation of standing waves or resonances while at the same time permitting the interior of the cap to function as a large acoustical capacity in an acoustical filter arrangement with the acoustical inductance provided by the sound input openings il in the mouthpiece. A second acoustical filter is provided in front of the mouthpiece 6 by the aperture It as an acoustical inductance, coupled to the acoustical capacity of the chamber 20. The dimensions of the aperture i8 and of the chamber 20 are such that the corrective acoustic filter thereby provided compensates for changes in the sound pickup introduced by the high impedance input, and a desired frequency response characteristic is obtained from the microphone in operation.

The creation of standing waves between the face of a user and the front of the microphone, is effectively prevented by the outer curved shape or contour of the dome or bullet shaped casing l which curves forwardly to the mouthpiece tip, and while not critical in shape, has been found to produce best results when smoothly rounded externally substantially as indicated in the draw- From the foregoing description it will be seen that an improved speech microphone is provided, in accordance with the invention, which permits close talking, that is, the application of high acoustical pressure and that such pressure may be applied without wind effects or turbulence within the microphone itself. Furthermore any suitable microphone may be used having a diaphragm of any, desired area, for the reason that the effective area of the diaphragm is determined by the annular mouthpiece and the input openings therein which are placed in the end of the dome shaped cap in spaced relation to the microphone. In accordance with the invention. furthermore, the high acoustical impedance of small openings and an arrangement which enables these openings to be placed where the sound level for speech is high has been effectively utilized creasing the ratio of speech input sound level to feedback or noise input sound level. Accordingly, it will be apparent to those skilled in the art that the invention provides an improved sound translating system in which acoustic feedback and acoustic noise pickup by a microphone is reduced to a minimum. 1

I claim as my invention:

, l. A. sound input device for a microphone including a diaphragm as one of the elements thereof, said sound input device comprising a casing having walls defining a cavity, said casing being provided with at least one aperture in a wall thereof for the passage of sound waves therethrough, said aperture having an area of a size which is relatively small compared to the diaphragm area of said microphone thereby to provide an acoustic resistance to sound waves within substantially such a predetermined frequency range as to provide a high speech sensitivity and a high speech to "ridis'e'input ratio in said range, means providing acoustic coupling between the microphone and said cavity, and acoustical damping material disposed within said cavity for introducing resistance to the formation of Standing waves therein.

2. The invention set forth in claim 1 characterized in that said acoustic coupling means is disposed in a wall of said casing opposite said first mentioned wall and comprises an acoustic filter disposed between the microphone and said cavity for modifying the sound waves applied to the microphone through said aperture.

3. The invention set forth in claim 2 further characterized in that said casing comprises an annular member with said first mentioned wall centrally and inwardly disposed in one end of said casing, and still further characterized in that an outer wind screen is provided on said one end being spaced from said central inner wall to provide a central frontal cavity therebetween, and wherein at least one opening is provided in said casing between said frontal cavity and the exterior of said casing for venting said cavity.

l. A sound input device for a microphone comprising a casing having walls defining an enclosure, acoustical filter means disposed in one wall of said casing and including an outer wind screen and an inner finely perforated wall in spaced relation to said screen for conducting sound into said enclosure, the space between said screen and wall being vented for the free flow of air therefrom laterally and in a direction normal to the flow of sound therethrough, and the perforated area of said wall being of a size to provide an effective diaphragm area for the microphone of reduced size, thereby to increase the ratio of speech level input to feedback and noise level input to the microphone.

5. A sound input device for a microphone comprising a casing having walls defining an enclosure, acoustical filter means disposed in one wall of said casing and including an outer wind screen and an inner finely perforated wall in spaced relation to said screen for conducting sound to the microphone, the space between said screen and wall being vented for the free fiow of air therefrom laterally and in a direction normal to the flow of sound therethrough, a wall of said casing opposite said finely perforated wall having an aperture therein and being arranged to receive the microphone, said aperture providing coupling between the microphone and the interior of said casing, and the perforated area of said first mentioned wall roviding an effective diaphragm area for the microphone of reduced size, thereby to increase the ratio of speech level input to feedback and noise level input to the microphone.

6. In a speech microphone unit, the combination of a microphone casing having a dome shaped cap, a speech input mouthpiece located at the tip of said cap and having a central frontal cavity and at least one opening of relatively small size communicating with the interior of the cap to provide a high acoustical input impedance for said unit, means for venting said mouthpiece comprising at least one lateral opening communicating with the outer end of said first named opening through said cavity, and a wind screen of relatively fine mesh providing a cover for said cavity.

7. The combination defined by claim 6, further characterized by the fact that the dome shaped cap is of an external contour to relieve standing waves tending to form adjacent. the

mouthpiece and that the interior of said cap provides a. chamber having a relatively large acoustical capacity as an acoustical filter element with the high acoustical impedance of the mouthpiece, still further characterized in that acoustical damping material is disposed within said dome shaped cap for introducing resistance to the formation of standing waves therein.

8. In a speech microphone unit the combination of a cylindrical cup-shaped microphone casing, a bullet-shaped hollow cap having a base end and an open tip end, said base end being seated on the face of said microphone casing to provide closure means therefor and having an opening therein connecting the interior of said microphone casing with the hollow interior of said cap, and a sound input mouthpiece for said microphone unit mounted on said bullet-shaped casing at the tip thereof, said mouthpiece comprising an annu ar hollow flange having an open face and extending forwardly of said tip to provide a central open cavity and having an inner wall disposed between said cavity and said 1101- low interior of said cap, said inner wall having at least one aperture therein of a size to provide high impedance sound communication between said cavity and the interior of said bullet-shaped cap and microphone casing, and a relatively finemesh screen disk seated in said annular flange and disposed in the open face thereof to provide wind screen closure means therefor, said annular flange having a plurality of radial ventilating openings extending laterally therethrough between said cavity and the exterior of said mouthpiece, and the area of opening in said inner wall of the mouthpiece being relatively small to provide an effective microphone diaphragm area of reduced size for the reception of speech in a region of relatively high sound pressure to the exclusion of noise pickup.

9. In a speech microphone unit the combination of a cylindrical cup-shaped microphone casing, a bullet-shaped hollow cap having a base end and an open tip end, said base end being seated on the face of said microphone casing to provide closure means therefor, means providing an acoustical corrective filter comprising an acoustical chamber within the microphone casing and a central aperture between said chamber and the hollow interior of said cap, a body of acoustical damping material within the hollow cap for introducing resistance to the formation of standing waves therein, and a sound input mouthpiece for said microphone unit mounted on said bulletshaped casing at the tip thereof, said mouthpiece comprising an annular hollow flange having an open face and extending forwardly of said tip to provide a central open cavity and having an inner wall disposed between said cavity and said hollow interior of said cap, said inner wall having at least one aperture therein of a size to provide high impedance sound communication between said cavity and the interior of said bullet-shaped cap and microphone casing, and a relatively finemesh screen disk seated in said annular flange and being disposed in the open face thereof to provide wind screen closure means therefor, said annular flange having a plurality of radial ventilating openings extending laterally therethrough between said cavity and the exterior of said 5 mouthpiece, and the area of opening in said inner wall of the mouthpiece being relatively small to provide an efiective microphone diaphragm area of reduced size for the reception of speech in a region of relatively high sound pressure to the 10 exclusion of noise pickup.

STEPHE N A. CALDWELL, JR.

REFERENCES CITED The following references are of record in the 15 file of this patent:

Number Number 8 UNITED STATES PATENTS Name Date Sonneborn July 2, 1912 Lynn Oct. 20, 1914 Spear June 27, 1916 Thomas July 6, 1920 Van Butters Dec. 18, 1923 Winckel Oct. 19, 1926 Whittaker May 18, 1937 Burroughs Feb. 10, 1948 FOREIGN PATENTS Country Date France Oct. 21, 1929 Australia June 23, 1938 Great Britain Sept. 4, 1936 

